The present application is related to the co-pending commonly assigned application of Tomii et al, Ser. No. 748,833, filed June 26, 1985. This co-pending application is directed to a flat CRT using a deflector of electric field type.
This invention relates to charged-particle beam deflectors and flat cathode-ray tube (CRT) using the same, which are used for displaying figures and TV pictures by way of charged-particle beam using an electron beam or ion beam which is deflected by an electric field.
It is well known that a charged-particle beam is refracted by an electric field to obtain deflection, and the principle of such deflection is disclosed, for instance, on page 101 of "Vacuum Tubes" written by K. R. Spangernberg published by McGraw-Hill in 1948. Various types of deflectors have hitherto been provided, and electron beam deflection electrodes or deflection plates used for sweeping an electron beam within a CRT, such as a TV picture tube or oscilloscope tube, are of this sort.
Two typical types of electron beam deflection electrodes are known, one being a simple parallel flat type, and the other being a curved type wherein the electrodes are curved along the orbit or path of a charged-particle beam for effective deflection. When a voltage is applied to such deflection electrodes, the orbit of the particles is bent in the direction of the electric field. The characteristic of deflectors is usually represented by the displacement of the travelling particles at the target where the particles reach after deflection. Namely, the displacement ys from the center of deflection electrode on the target is given, as shown on page 412 of the abovementioned "Vacuum Tubes", by: ##EQU1## wherein Vo is a voltage [V] for accelerating particles entering the deflector;
Vd is a voltage [V] applied across deflection electrodes; PA1 a is a distance [m] between the facing deflection electrodes; PA1 b is a length [m] of the deflection electrodes measured in a direction of nondeflected beam path; and PA1 l is the distance [m] between the center of the deflection electrodes and the target;
The voltage Vd required for deflection is referred to as a deflection voltage. It is preferable in practice to obtain a larger deflection displacement ys by using less deflection voltage. To this end, it is necessary for (1) distance "a" between a pair of facing deflection electrodes to be small and the length "b" of the deflection electrodes and the distance l between the center of the deflection electrodes and the target to be large or the acceleration voltage Vo of the particles to be low. Since deflected particles would collide with the deflection electrodes if the distance "a" between the pair of deflection electrodes is small and the length "b" of the same is large, there is a limit in each of these sizes. Similarly, when the distance l between the deflection electrodes and the target is large, it results in large-sized equipment; for instance, in the case of a CRT for a TV, the depth of the entire structure would be very large. Nextly, while it is effective to reduce the acceleration voltage Vo, this acceleration voltage Vo is defined usually in view of the utilization of particle beam. For instance, in the case of a CRT for a TV set, since the fluorescent material painted on the target is excited by the cathode ray, i.e. the electron beam, to cause the material to emit light so as to display an image, the acceleration voltage Vo must have a high value which is over 1.0 kV in order to obtain necessary luminance. To solve this problem in conventional arrangements, a post accelerating system has been used. More specifically, the particles travel at a low speed when passing the deflection electrodes to receive sufficient deflection force, and the particles are accelerated after passing the same during travelling to the target. However, with this method the electron path is bent radially inwardly due to the lens operation caused by the disturbance of the electric field at the outlet of the deflection system and due to the accelerating electric field applied thereafter, and thus it is difficult to obtain high deflection sensitivity. In order to improve this, a mesh is located at the outlet of the deflection system to avoid the disturbance of electric field, but there have been various drawbacks such that the utilizing rate of the beam is reduced, secondary electrons are generated, disturbance of beam focusing occurs and so on.
As described above, in the conventional arrangement, in order to obtain large deflection displacement, namely to obtain sufficient deflection sensitivity, using a lower deflection voltage, it is necessary that the deflection voltage be high, the length of the deflection electrodes be long, or a post accelerating system be used. However, a high deflection voltage results in high deflection power while there are some limits caused by circuit techniques, and, therefore, lengthening the deflection electrodes suffers from the problem of particle collisions with the deflection electrodes and from the depth-increase problem. Furthermore, with the post accelerating system, there is the problem of undesirable images being displayed in addition to the above-described various problems of deflection sensitivity relating to electric field disturbance at the outlet of the deflection system and to the occurrence of the secondary electrons and reduction in utilization rate of the electron beam due to the mesh.